The invention relates to a multilayer coating having reflection properties in the infrared and/or in the solar radiation range, particularly of the low-emissivity type, for transparent substrates, particularly windows. This type of multilayer coating comprises a lower dielectric antireflection treatment layer, at least one silver-based functional layer, preferably at least one metallic protection layer deposited on and/or beneath the silver layer, as well as an upper antireflection treatment layer comprising several partial dielectric layers.
In the context of the invention, the lower dielectric antireflection "layer" is composed of one layer or of a superposition of at least two layers of dielectric material, of the metal oxide or nitride type (such as, for example AlN or Si.sub.3 N.sub.4) or alternatively of a metal oxnitride or silicon oxynitride. Likewise, the upper antireflection "layer" comprises a sequence of several layers based on a dielectric material of the metal oxide type, but it may also comprise other dielectric layers of the nitride or oxynitride type mentioned above.
Low-emissivity multilayer coatings of this kind are known in various forms. They are generally manufactured using a magnetic-field-assisted sputtering process, the metal oxide layers obtained from metal targets being deposited reactively using a reactive gas containing oxygen (and nitrides using a reactive gas containing nitrogen). The metallic protection layers of a metal having a relatively higher affinity for oxygen, immediately adjacent to the silver layer or layers, serve to protect the silver layer from the oxygen which gets in by diffusion, not only during the subsequent process of reactively sputtering the antireflection treatment layer but also during any later heat treatment and during use, depending on their function in coated windows.
As a general rule, coated windows are stacked one on top of another in the form of large packets and are transported over great distances, placed in a slightly inclined position on suitable transportation mountings. In the case of prolonged travel, the layers are exposed to particular mechanical stresses because of vibrations. These mechanical stresses may result in the multilayer coating being damaged, the damage generally being in the form of visual defects of the scratching or scoring kind.
It is knows that multilayer coatings having an "upper" antireflection treatment layer (on top of the functional layer(s) of the silver type), which is composed only of a single metal oxide, for example SnO.sub.2, ZnO, TiO.sub.2, Bi.sub.2 O.sub.3 or Al.sub.2 O.sub.3, are relatively sensitive to mechanical and chemical stresses. This is why various suggestions have been made for producing the antireflection treatment layer in the form of a multilayer structure, for the purpose of improving the chemical and mechanical resistance of multilayer coatings of this kind.
A multilayer coating having improved resistance to environmental conditions and to mechanical stresses is known from document EP-0,593,883 B1. In the case of this known multilayer coating, the upper antireflection treatment layer is composed of a non-metallic triple layer, preferably having two layers of identical chemical composition, zinc oxide and titanium dioxide being preferably sputtered alternately. In this case, a layer of zinc titanate is formed during the coating process, this layer being in the subnanometre range and reinforcing the protecting action of the metallic protection layer placed on the silver layer with respect to environmental influences. Furthermore, a non-metallic surface layer made of TiO.sub.2 is preferably placed on the non-metallic triple antireflection treatment layer.
The manufacture of a multilayer coating of this kind, using titanium dioxide layers, is relatively complicated because the titanium dioxide layers can be manufactured only at a relatively low sputtering rate. What is more, it is necessary to provide in the continuous coating plant, for the purpose of applying the triple antireflection treatment layer, at least three cathode positions for the antireflection treatment layer, these not always being present in existing coating plants.
In order to improvre the scratch resistance of the low-emissivity multilayer coating, it is also known to deposit, on the upper antireflection treatment layer, a thin surface layer made of a material of high hardness, particularly Si.sub.3 N.sub.4, SiO.sub.2 or TiO.sub.2. Suggestions of this kind are known, for example from the already mentioned document EP 0,593,883 B1, as well as from document WO 95/29883 and document DE 19530331 A1. However, it has turned out that multilayer coatings, even those provided with hard surface layers of this kind, do not in any way satisfactorily improve the transportation behaviour, at least when the antireflection treatment layer deposited under this surface layer is relatively thin, namely less than 30 nm, which may, however, be necessary in order to obtain defined colours in transmission and/or in reflection. Despite a marked improvement in other mechanical properties, the mechanical damage described is also observed, such as scratching and scoring, even in the case of multilayer coatings provided with a hard surface layer, when packets of windows thus coated are exposed to transportation vibrations during prolonged travel.